Electric cars are an axiom of clean transport planning - they produce no tailpipe emissions, little localised air pollution and, potentially, no greenhouse gas output. But as their critics point out, they are only as green as the electricity that they use.

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A power supply dependent on fossil fuels will produce greenhouse gas emissions from electric vehicles that are less than - but still comparable to - those from automobiles fitted with internal combustion engines (ICE)

One recent study by environmental scientists at the International Council for Clean Transportation (ICCT) projected that by 2015, a fully-electrified Nissan Leaf would emit 20 grams of CO2 (g/km) if driven in nuclear energy-reliant France, but 114 g/km in the decidedly less green fields of the UK.

A separate research paper by the European Association for Battery Electric Vehicles estimated CO2 emissions from a plug-in vehicle charged in coal-dependent Poland or Luxembourg at approximately 130 g/km — the same as the EU standard for cars with ICE in 2015.

But in the US and China, the ICCT estimates that dirty fumes from a leaf would be even worse, at 136 g/km and 182 g/km respectively.

Such figures are not widely understood, said Greg Archer, the clean vehicles spokesman for the Transport and Environment pressure group.

"There has undoubtedly been some hype about the short-term potential of electric vehicles," he said, "but that is not to say that in the longer term they will not prove to be a very successful technology."